1. Field of the Invention
The present invention relates generally to intraluminal ultrasonic measurement, and, more particularly, but not by way of limitation, to devices and methods for measuring reflux volume during spontaneous gastroesophogeal reflux (GER) events.
2. Description of Related Art
Recent statistics provided by the National Institutes of Health, suggest that symptoms consistent with Gastroesophageal Reflux Disease (GERD) resulted in 710,000 hospitalizations in the USA for 2002 [1]. GERD is generally caused by abnormal retrograde flow of gastric content into the esophagus, and may result in various symptoms such as mucosal damage [2]. GERD is one of the most common conditions that affect the gastrointestinal tract, and is usually thought to be the cause of many esophageal symptoms [10]. Individuals suffering from GERD may experience symptoms such as heartburn or a substernal burning sensation in the chest [3]. In addition, untreated GERD may lead to further consequences, such as esophagitis and Barrett's esophagus, which has been considered a precursor to esophageal cancer [4, 5].
Various techniques have been devised to diagnose GERD, but methods for quantifying the volume of a spontaneous reflux event are currently lacking [11]. Presently, three techniques have been attempted to monitor the volume of reflux, namely, (a) video-fluoroscopy; (b) combined ambulatory multichannel impedance-pH monitoring and (c) high-frequency intraluminal ultrasonic methods [11].
Video-fluoroscopy is an imaging technique using X-rays to obtain real-time dynamic video of the internal organs of a patient. This technique may be indicated for patients with suspected aspiration, to detect swallowing dysfunction, gastroesophageal reflux, and achalasia [15]. While anatomic and functional information related to GERD might be provided by this stationary method, non-physiological conditions are required to induce gastroesophageal reflux phenomena during testing [11-14]. In addition, fluoroscopy usually involves exposure of the subject to significant amounts of ionizing radiation, which limits the duration of the test and practical feasibility for detecting spontaneous reflux events. Thus, video-fluoroscopy and other similar radiology methods (e.g. esophageal scintigraphy [12]) are generally not suitable for monitoring spontaneous reflux episodes.
Multichannel impedance-pH monitoring has been developed for gastroesophageal reflux detection [6, 14, 16-18]. As explained by Lazarescu and Silny [14, 16], this technique is generally based on measuring electrical impedance between electrodes mounted on a thin intraluminal catheter. If the electrodes are in contact with content of high ionic concentration, e.g., swallowed or refluxed material, low impedance is measured. Correspondingly, if they are in contact with content of low ionic concentration, e.g., air or esophageal lining, high impedance is measured. Also, impedance changes in the temporal-spatial patterns at different positions on the catheter within the esophagus facilitate the differentiation between antegrade and retrograde bolus movement. U.S. Pat. No. 5,833,625, entitled Ambulatory Reflux Monitoring System, describes a system for monitoring reflux based on impedance changes in the esophagus. U.S. Pat. No. 4,119,498, entitled Monocrystalline Metal Electrode and Method of Use, describes a metal electrode for determining pH.
Multichannel impedance-pH monitoring may have a high sensitivity in detecting the proximal extent of reflux and the duration of acid clearance [6, 14]. However, the correlation between the volume and the proximal extent of the reflux may be adversely affected by several specific factors, such as specific characteristics of the refluxed gastric content, the compliance of the gastroesophageal junction, and the gastroesophageal pressure gradient [6, 19-20]. Additionally, as described by Srinivasan [21], similar decreases in impedance system output were observed when different volumes of liquid boluses were swallowed by the same subject.
U.S. Pat. No. 6,398,734, entitled Ultrasonic Sensors for Monitoring the Condition of Flow through a Cardiac Valve, discloses a band with two ultrasonic transducers to be wrapped around a cardiac vessel in order to evaluate the shape of the vessel and the velocity of the blood flow in the vessel. U.S. Pat. No. 5,247,938, entitled Method and Apparatus for Determining the Motility of a Region in the Human Body, describes an apparatus using ultrasonic probes for monitoring the movement of the digestive tract wall. This objective was accomplished by attaching several ultrasonic probes to a region of the intestinal wall and processing the echo signals. The systems disclosed in these two patents are not catheter-based, and generally require complicated and/or invasive setup operation procedures.
U.S. Pat. No. 4,802,490, entitled Catheter for Performing Volumetric Flow Rate Determination in Intravascular Conduits, presents an apparatus for determining volumetric information of vascular conduit using an intraluminal catheter. A transverse ultrasonic transducer and an inflatable balloon are positioned on the distal portion of the catheter. The ultrasonic signal is connected and processed by a Doppler circuit to record blood velocity both when the balloon is inflated and deflated. Thereafter, the internal cross-sectional area of the blood vessel is calculated based on the assumption that the cardiac output is constant regardless whether the balloon is inflated or not. However, this assumption is not adequate for measuring the volume of reflux which changes temporally and frequently.
Intraluminal ultrasonic imaging catheters have been used to measure the cross-sectional area of the distal esophagus during distensions by bolus swallows or spontaneous reflux events [6, 14]. Compared to distal distension in normal subjects, larger distensions of the esophagus were found in patients with GERD symptoms [7], [8]. An example of such ultrasonic methods is described in U.S. Pat. No. 4,417,583, entitled Apparatus and Method of Internal Examination of Gastrointestinal Tract and Adjacent Organs, which presents an ultrasonic imaging method for examining an organ of the gastrointestinal tract. Current intraluminal ultrasonic imaging systems are generally limited to stationary studies and require complicated test procedures. Moreover, due to their use of cumbersome image processing, these systems are not appropriate for 24-hour ambulatory monitoring or detecting spontaneous reflux events [8].